1. Field of the Invention
This invention relates to failure detection systems for hydraulic pumps which are widely in use as a source of power for operating hydraulic excavators, hydraulic cranes and other hydraulic equipment and machinery, and more particularly it is concerned with a failure detection system for a hydraulic pump of the type described which is equipped with a displacement volume varying device and connected to at least one hydraulic actuator to constitute a hydraulic circuit for driving the hydraulic actuator.
2. Description of the Prior Art
A hydraulic pump which is used with a hydraulic excavator, a hydraulic crane and other hydraulic equipment and machinery constitutes the most important means for producing hydraulic energy, and a reduction in its performance due to failure or changes with time poses a serious obstacle to the operation of equipment and machine relying on the hydraulic pump for a supply of power. It is thus imperative that the operation of a hydraulic pump be checked to see if it is properly functioning. A failure detection system of the prior art conventionally used to check on the hydraulic pump to see if the pump shows any sign of failure or deterioration in performance (hereinafter inclusively referred to as failure) will be described.
Such failure detection system of the prior art comprises a hydraulic pressure tester connected to a discharge line of a variable displacement type hydraulic pump equipped with a displacement volume varying device (typical of which is a swash plate to which reference will hereinafter be made), and a regulator for actuating the swash plate in accordance with the discharge pressure of the hydraulic pump. The hydraulic pressure tester comprises a pressure gauge for measuring the hydraulic pressure, a flow meter for measuring the flow rate of a hydraulic fluid, and a manually operable variable restrictor for throttling the discharge of the pump to raise the discharge pressure. The variable displacement type hydraulic pump has connected thereto a revolution counter for measuring the number of revolutions thereof.
The operation of the failure detection system of the aforesaid construction will be described. A hydraulic fluid line which is connected to the discharge port of a variable displacement type hydraulic pump to be checked and constitutes part of a hydraulic circuit in which the pump is connected to at least one hydraulic actuator is cut off in a position close to the pump, and the hydraulic pressure tester is connected to the cut end of the line. Then, the pump is driven by an engine or other prime mover, and the number of revolutions N of the pump is measured by the revolution counter. While the pump is thus being driven, the variable restrictor of the tester is actuated to throttle the flow through the line until the hydraulic pressure indicated by the pressure gauge (the discharge pressure of the pump) becomes equal to a set pressure value P.sub.ref. At this time, the flow rate of the discharged hydraulic fluid Q from the pump is measured by the flow meter. The flow rate of the discharged hydraulic fluid Q should vary depending on the magnitude of a shifting or tilting of the swash plate which is controlled by the regulator in accordance with the discharge pressure. Thereafter, a theoretical flow rate of the discharged hydraulic fluid Q.sub.ref from the pump is calculated based on the number of revolutions N and the set pressure value P.sub.ref. Finally, the theoretical flow rate of the discharged hydraulic fluid Q.sub.ref is compared with the flow rate of the discharged hydraulic fluid Q measured previously, and the pump is diagnosed to be out of order when the result of the comparison exceeds an allowable value.
Some disadvantages are associated with the failure detection system of the prior art of the aforesaid construction and operation. For one thing, when a pump is checked, the hydraulic pressure tester must be connected to a portion of a hydraulic fluid line by cutting it off the rest of the line. This operation is time-consuming and has the risk of dust and other foreign matter being incorporated in the hydraulic fluid flowing through the line. For another thing, checking consists in actuating the variable restrictor and reading the pressure gauge and flow meter. This operation is also time-consuming and troublesome. Hydraulic machines and apparatus of a large size, such as a hydraulic excavator, are equipped with a multiplicity of hydraulic pumps. Thus, when the failure detection system of the aforesaid construction is used for checking the hydraulic pumps, difficulties have been experienced in quickly locating the failed pump.